The present invention relates to a power assisted steering device comprising a rotatable steering column composed of two aligned parts fastened respectively to a steering control means and to a steering operating means, the said parts having relative angular movability limited by abutment means and being connected together by a torsion bar, and further comprising power assistance means which come into action in response to an angular movement of the said parts in relation to one another and acting in the same direction as the operating means in order to reinforce the action of the latter.
The invention relates more particularly to a power assisted steering device of this kind for automobile vehicles in which the control means is a steering wheel, and the operating means is a pinion meshing with a rack. The latter controls the steered wheels of the vehicle with the aid of connecting rods. The assistance means comprise a hydraulic cylinder acting on the rack and fed from a central hydraulic system with the aid of a distribution means sensitive to the relative angular position of the two parts of the steering column.
As will be understood, the torsion bar connecting the two parts has the effect of returning these parts to and holding them in a relative angular position of rest. When the driver turns the steering wheel the torsion bar undergoes deformation and transmits the torque to the operating means, but at the same time the angular displacement of the two parts in relation to the position of rest frees a hydraulic flow which causes the cylinder to act on the rack.
The abutment means disposed between the two parts are intended to ensure safety in the event of mechanical failure, but do not intervene in normal operation. For example, the torsion bar suitably permits normal angular displacement of the two parts of the order of 1.degree. in either direction, whereas the angular movability of the two parts between the two limit positions defined by the abutment means is of the order of 6.degree..
The torsion bar thereby constitutes an element which is essential to the sensitivity of control for the purpose of bringing the assistance means into operation, while safety is ensured by the abutment means.
Up to the present time, in known devices the different purposes served on the one hand by the torsion bar, that is to say sensitivity, and on the other hand by the abutment means, that is to say safety, had not generally been taken into account.
In such known devices, the torsion bar usually has a shape consisting of a rod provided with two end heads of larger diameter for the purpose of connection by pins to the two parts of the column.
An arrangement of this kind has numerous disadvantages. Firstly, through their very existence the presence of the heads entails a large diameter. Secondly, connection by pins is inconvenient if it is borne in mind that one of the heads must be fastened to one of the parts by drilling in situ with accurate angular control. Furthermore, the presence of these pins makes it necessary to take special precautions in relation to sealing.
The main object of the present invention is the provision of a power assisted steering device of the type indicated above, which is free from the disadvantages indicated and whose construction is simple and convenient.